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Decoupling capacitor selection

P

PeteS

Jan 1, 1970
0
I've had chips where the mfr has requested I put those low inductance
vias to the ground and power planes *in* the device pads.

I didn't, and everything worked fine, although I did take great care
with the decoupling. Obviously, that sort of thing has implications at
board reflow.

On another much earlier point, caps up to 100uF are available in
multilayer ceramic (although the 100uF are 6.3V, X7R,1210) which I have
used extensively with no problems. To my understanding, the capacitor
manufacturers are trying to increase the capacitance available, (with a
generally lower voltage range) for low voltage, high current systems.

Cheers

PeteS
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that PeteS <[email protected]>
wrote (in said:
To my understanding, the capacitor manufacturers are trying to increase
the capacitance available, (with a generally lower voltage range) for
low voltage, high current systems.

They've been doing that since around 1920. Equipment manufacturers
always want more uF in fewer m^3.
 
J

Joerg

Jan 1, 1970
0
Hello Pete,
I've had chips where the mfr has requested I put those low inductance
vias to the ground and power planes *in* the device pads.

I have done that. But the assembly house folks are going to throw the
pots and pans at you for that.
On another much earlier point, caps up to 100uF are available in
multilayer ceramic (although the 100uF are 6.3V, X7R,1210) which I have
used extensively with no problems. To my understanding, the capacitor
manufacturers are trying to increase the capacitance available, (with a
generally lower voltage range) for low voltage, high current systems.

Past 1uF they tend to get expensive. On one design I had an electrolytic
in there and someone changed it to ceramic. Unfortunately that was at
the output of an LDO regulator and sure enough that LDO started singing
the blues. It wanted an ESR below x _but_ above y. I guess that's the
last time I have designed in an LDO regulator. Many of these things are
IMHO the pits. The amazing thing was that the LDO produced a sawtooth
from 3V to 5V and the board worked anyway with a precision this was only
slighty reduced. But it drowned out Rush Limbaugh on the radio...

Regards, Joerg
 
P

PeteS

Jan 1, 1970
0
<< On one design I had an electrolytic in there and someone changed it
to ceramic. Unfortunately that was at the output of an LDO regulator
and sure enough that LDO started singing the blues.>>

I wrote on this issue on a thread some time ago on the issues of
ceramics at the output of a switcher or an LDO. It can cause grief
untold (as you found). It can be *very* difficult to get a switcher or
LDO to operate properly with ceramics on the output within perhaps 3 cm
of track (the track inductance helps to damp the low esr effect, of
course). One trick I use is a 1 ohm resistor in series with the ceramic
- seems to work well.
One can only be glad it drowned Mr. Limbaugh - I used to refer to him
as the definition of Oxymoron - self definitional, I think ;)

Cheers

PeteS
 
J

Joerg

Jan 1, 1970
0
Hello Pete,
I wrote on this issue on a thread some time ago on the issues of
ceramics at the output of a switcher or an LDO. It can cause grief
untold (as you found). It can be *very* difficult to get a switcher or
LDO to operate properly with ceramics on the output within perhaps 3 cm
of track (the track inductance helps to damp the low esr effect, of
course). One trick I use is a 1 ohm resistor in series with the ceramic
- seems to work well.

Actually I never had a problem with ceramics or for that matter any
capacitors on switcher outputs. Next time I am squeezed for dropout
voltage I am just going to design in a switcher. Then I know it works.

Regards, Joerg
 
P

PeteS

Jan 1, 1970
0
The esteemed Joerg wrote:
<< Actually I never had a problem with ceramics or for that matter any
capacitors on switcher outputs>>

Nor I - if that is what I designed in the first place :)

I had to 'redo' a design that was clearly not right - and that was
difficult; indeed it proved impossible. The client insisted I could not
change the layout, which made it almost impossible as the device used
was designed for 1A (limited by the device used to a great extent) and
the load could have 3A pulses. I eventually told the client that what
was required (no layout changed and power did not deviate) was not
possible.

He wasn't happy - oh well.

Cheers

PeteS
 
J

Joerg

Jan 1, 1970
0
Hello Pete,
The esteemed Joerg wrote:

Nice. said:
I had to 'redo' a design that was clearly not right - and that was
difficult; indeed it proved impossible. The client insisted I could not
change the layout, which made it almost impossible as the device used
was designed for 1A (limited by the device used to a great extent) and
the load could have 3A pulses. I eventually told the client that what
was required (no layout changed and power did not deviate) was not
possible.

He wasn't happy - oh well.

With clients it's often the same as with patients. If someone with a
hardcore cholesterol issue is told by the doctor to cut out the cheese
burgers and then says that would be impossible, oh well.

Regards, Joerg
 
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